Current DNA sequencing is done mostly by Sanger methods and other sequencing-by-synthesis methods. These methods suffer from high cost, short read lengths, and insufficient throughput.
Sequencing by electron microscopy has also been explored. The idea of sequencing by electron microscopy is not new. It was proposed by Richard Feynman only six years after the structure of DNA was discovered. However, it has never been successfully used to generate meaningful sequence information.
The transmission electron microscope (TEM) works by sending an electron beam through a sample and onto a detector or screen. Portions in the sample impede or deflect the beam, so that the pattern of electrons reaching the detector forms an image. In most situations, atoms of low atomic number (Z) produce very little contrast and are essentially invisible in the electron microscope. Ordinary DNA, comprising low-Z hydrogen, carbon, nitrogen, oxygen, and phosphorus atoms shows almost no contrast in an electron microscope and is almost impossible to see against a supporting background. To visualize DNA using current electron microscopy techniques, the bases may be labeled with high-Z atoms or otherwise rendered detectable by TEM.
The first serious work on sequencing by electron microscopy was done by Beer, M. and Moudrianakis, E. N., 48(3) PNAS 409-416 (1962). His initial work focused on heavy atom labels for DNA and also attempting to visualize heavy atoms in the electron microscope. Later work, and probably the best other work to date, was done by Whiting and Ottensmeyer. It was reported by Ottensmeyer:                Heavy atom markers for thymine and adenine and guanine were developed and tried on model sequences, only to indicate that although the electron microscope and the chemistry were no longer major obstacles, specimen preparation was. The uncontrollable placing of a marked single-stranded nucleic acid polymer on the specimen support resulted in a rather non-uniform base-to-base spacing. Therefore an easy and accurate reading from a single molecule was not possible.” Ottensmeyer, F. P. (1979). “Molecular Structure Determination by High Resolution Electron Microscopy.” Ann. Rev. Biophys. Bioeng. 9129: 129-144. (Internal references omitted.)        
There is great biomedical importance to having the ability to rapidly sequence individual genomes and other sequences, and as described above, improved methods are needed.